Kurt,
AP cartridges/bullets are military armor piercing cartridges that have a hardened core (I have used the cores from both 30 and 50 caliber AP bullets as center punches). They are not as common now as they were 50 years ago when WW2 surplus ammo was being released. 30-06 AP ammo was accurate enough that it was used as substitute match ammo from time to time. Liberals didn't like it, claiming it was used for "cop killing" although I don't recall any incident of that happening.
Mike